This invention relates to the float process for forming flat glass where molten glass is continuously delivered onto one end of a pool of molten metal, where it is drawn to a ribbon of desired thickness and withdrawn as a continuous ribbon of glass from the opposite end of the pool of molten metal. More particularly, this invention relates to improvements in the delivery section of such an apparatus.
In the various types of float forming processes used to manufacture flat glass, each employs a delivery section wherein molten glass from a melting furnace passes from contact with a refractory vessel into contact with the molten metal of the forming chamber. The final piece of refractory contacted by the glass in this zone has been variously termed the "lip" or "lipstone," particularly in association with the delivery arrangement shown in U.S. Pat. Nos. 3,083,551 and 3,220,816 of Pilkington, and as the "threshold" in connection with the delivery system of the type shown in U.S. Pat. No. 3,843,346 of Edge et al. The term "threshold" will be employed herein to include all such final refractory delivery pieces.
A threshold is inherently subject to erosion due to the continuous movement of molten glass over its surface at a relatively high velocity. However, erosion of a threshold can negatively affect the optical quality of the glass being produced. Therefore, in spite of efforts to employ durable, high quality refractories for the threshold, it is frequently necessary to replace a threshold before the need for other repairs justifies shutdown of a melting and forming installation. Shutting down such an installation can be very costly in terms of lost production since cooling and reheating each requires several days, and a return to stable production can sometimes require several weeks overall. Therefore, threshold changes are sometimes carried out without shutting down the melting furnace by blocking the flow of molten glass to the delivery zone. However, in order to enable maintenance workers to remove brickwork around the delivery zone and to engage the threshold with hoist means, it has heretofore been considered necessary to cool the float forming chamber. Unfortunately, this results in an excessive loss of production time since cooling and reheating the forming chamber requires much more time than is required for the replacement of the threshold. Accordingly, it would be highly desirable if a threshold could be removed and replaced while the forming chamber remains at substantially operating temperature.